(1) No amount of rising air will produce precipitation
unless moisture is present. The more moisture that is present, the higher the potential for precipitation if
uplift mechanisms are in place. In order for
thermodynamic thunderstorms to occur, there must be moisture in the low levels
of the troposphere (between the surface and 700 millibars). A
dewpoint of 55 degrees in used as a critical value for
having enough moisture to sustain a thunderstorm. Thunderstorms generally do not occur when the surface dewpoint is
less than 55 degrees F (exception: thunderstorms/snow/sleet due to
elevated convection). This is why thunderstorms are not common well
behind
drylines or cold fronts. Most thunderstorms occur in the warm sector of a mid-latitude cyclone or due to strong
low level
instability (summer time air mass thunderstorms). Warm air has the capacity to contain more water vapor.
Higher concentrations of water vapor release abundant
latent heat.

(2) Precipitation amounts are shown on the synoptic
scale forecast models. Part of the formula for determining precipitation amounts is the quantity of moisture in the
air. The quantity of moisture is found by summing the actual mixing ratios from the surface to the upper levels of
the troposphere. Keep in mind the synoptic models can not handle mesoscale precipitation well. The synoptic models
depict precipitation over a large region but can not predict mesoscale rainfall gradients or precipitation amounts
at a point location with a high degree of accuracy especially when precipitation is
convective.

(3) Cloudiness without precipitation will result when enough
uplift is present to saturate the troposphere, but not enough uplift to condense out raindrops that can reach the
surface. When a forecast model has positive
UVV over the forecast region with a 70% or higher
850-500 average RH,
cloudiness becomes
likely. If RH is lower, UVV will need to be higher to develop clouds. If UVV is 3 or greater, and RH is 70% or greater,
expect synoptic scale cloud development. The
PBL moisture is very important. If the PBL is
dry, UVV will have to
increase for precipitation to reach the surface compared to a moist PBL.

(4) The dewpoint can be used to forecast temperatures. In a
barotropic troposphere (no fronts around), the
low temperature tends to be close to the evening dewpoint (especially when dewpoints are
above 60 degrees F) on a night with clear skies and wind less than 15 mph. A high dewpoint can limit
afternoon warming due to
evaporative cooling from the soil, vegetation, etc.

(5) Moisture convergence is the advection of moisture into a fixed region. Moisture convergence
results in increasing low level dewpoints, the potential for heavier rain if rain occurs, and a higher value of
Theta-E. Moisture advection can cause rainfall totals to be higher than the
Precipitable Water value given on a
thermodynamic diagram because moisture is continually transported toward a fixed location. Moisture advection is
common along cold fronts or any other low-level boundary that causes general
convergence. Moisture advection is
common when a mid-latitude cyclones transports moisture from a warm water body within the warm sector of the
mid-latitude cyclone.

(6) The drying power of the air can be estimated using the RH or
dewpoint depression. A
low RH in warm air or a large dewpoint depression in warm air (they both go hand in hand) will produce a large
drying potential. Soil and vegetation will dry quickly in this situation. Precipitation falling into dry air
will lose some or all of its mass to evaporation.